support pulling images with OCI Distribution Spec API

@cyphar Any reason not to have support for pulling OCI images directly without skopeo?

e.g. 
```
$ skopeo copy docker://alpine:latest oci:/tmp/alpine:latest
$ umoci unpack --image /tmp/alpine /tmp/alpine-bundle
```
could be written as 
```
$ umoci unpack --image docker://alpine:latest /tmp/alpine-bundle
```